Piezoelectric loudspeaker



Dec. 2l, 19.37.

S, BALLANTINE PIEZOELECTRIC LOUDSPEAKERl 2 Sheets-Sheet l Filed June 14,1933 lll/111111111111 Dec. 21, 1937.

S. BALLANTlNE PIEZOELECTRIG LOUDSPEAKER 2 Sheets-Sheet 2 Filed June 14,1933 N n m M v. a fr WL wb wn.. r. v N1. 4,

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Patented Dec. 21, 1937 PIEZOELECTRIC LOUDSPEAKER -Stuart Ballantine,Mountain Lakes,fN. J., assignor, by `mesme assignments, to RadioCorporation of America, New York, N. Y., a corporation of DelawareApplication June 14, 1933, Serial No. 675,834

11 Claims.

This invention'relates to electroacoustic apparatus for the conversion.of electrical energy' into sound energy, and more particularly toelectroacoustic apparatus of the type employing a piezoelectricallyactive crystal assembly.

An object of the invention is to provide electroacoustic apparatus ofsimplified constructionl and in which the active element is apiezoelectrically active diaphragm comprising a plurality of units, eachunit comprising two crystal plates which have a torque response when apotential is impressed across the electrodes associated with the crystalplates. An object is to provide a loud speaker of the horn type, anddriven by a piezoelectrically active crystal assembly of the typestated. A further object is to provide a loud speaker'of the typestated, and in which the circuit or circuits supplying power to thespeaker compensate for inequalities in the sound pressure-frequencyresponse characteristic of the reproducer elements, thus rendering thecombined characteristic of the electrical supply circuit and thereproducer elements more nearly uniform over the band of frequencieswhich is to be reproduced.

A further object is to produce a loud speaker of the above type which isparticularly adapted for the reproduction of the higher audiblefrequencies.

' These and other objects of the invention will taken with theaccompanying drawings in which: Fig. 1 is a longitudinal central sectionthrough a loud speaker embodying the invention, the sectionl being takensubstantially on line I-l oi Fig. 2;

Fig. 2 is a transverse section substantially on line 2-2 of Fig. 1;

y Figs. 3a and 3b are somewhat schematic views, in perspective, ofcrystal shear sensitivity;

Fig. 4 is a similar view of the crystal plate diaphragm system of thespeaker shown in Fig. 1;

Fig. 5 is a curve showing the sound -pressure- .frequency characteristicof a small piezoelectric loud speaker designed for the reproduction ofthe higher audio frequencies;

Figs. 6a, 6b and 6c are circuit diagrams of' the loud speaker andassociated circuits for supplying audio frequency voltages thereto, thecircuits including inductance for altering the sound pressure-frequencycharacteristic; and h Fig. 7 is a curve sheet showing a series of curveswhich illustrate the compensating eifect be apparent from the followingspeciiication when plate systems having obtainable with a supply circuitsuch as shown in Fig. 6b.

In the drawings, the reference numeral l identifies a crystal platediaphragm assembly which is clamped to the base of the horn 2 by meansof screws 3 which bear against the annular metal ring 4, a compressiblegasket 5 being arranged between ring 4 and the crystal assembly, properclearance between the crystal faces and the horn base -being provided bythe annular spacer 6. The spacer ring B may be made of rubber or of anyother suitable material capable of forming a seal to prevent the escapeof air from the throat chamber 1. 'I'he throat of the horn is ofuniformdiameter out to the tip of the insert 8 whichvis approximatelybullet-shaped to provide the `de sired exponential law of the increasein the crosssection of the horn.

The insert 3 provides an annular throat passage at the base of the horn,and thus reduces the average length of the paths from the rvarious partsof the crystal diaphragm to the throat,

thus bringing the phases of the air currents due The crystal diaphragm[comprises iour square crystal plates cemented together at their edges.to form a square assembly, as illustrated in Fig. 4. 'I'he method ofoperation of this assembly will be best understood by rst consideringthe simpler crystal plate systems shown in Figs. 3a

and 3b.

The plates A are cut from homogeneous Rochelle salt crystals with the atfaces of the crystals normal Ato the electrical or a axis and thesidesparallel to the b and c axes. Electrodes B of metal foil arecemented to the faces of the y crystals and, as is well known, theapplication ofa potential difference between the electrodes tends tostrain the crystal. When the cornersy of the crystals are heldimmovable, the application of direct current potentials o! thepolarities indicated will tend to move the upper edges of the crystalsin shear in the directions indicated by the arrows adjacent those edges.

ward'the other tends to move backward, the

result will be a twisting or torque motion of the upper part of theassembly when the corners 1:, il are clamped. When one of the uppercorners e is also clamped, the free corner d will move at right anglesto the plane of the crystal plates, as shown by the arrow D. This methodof securing 4torque sensitivity fromjshear plates is described by C.Baldwin Sawyer in The Proceedings of the Institute of Radio Engineers",Nov.

Y 1931. page 2025.

The crystal plate diaphragm of the present invention comprises fourunits of the type shown in'Fig. 3, these units being cemented togetherat their 4edges to form the squa're assembly which is illustrated inFig. 4. When this assembly is clamped to the horn base by the ring l andthe screws I, the corners z, 1,' and z of each doubleplate unit of thecomposite diaphragm are sub- V stantially prevented from moving and theapplication of a potential diiference between the electrodes causes thefree corners d to move in and out inunison in a direction normal to theplane of the assembly. 'I'he Awhole assembly thus acts as a diaphragmand functions in the same manner for the generation of sound as theordinary type of horn loud speaker.

ToV prevent the undesired escape or entrance of air into the throatchamber, the union oi' the four crystal plates at their edges must becare- Y fully made so as to remain air tight. If cement is used, it ispreferably of a plasticized resin type, such as cellulose nitrate cementcontaining castor oil, which will remain resilient after setting toavoid any constraint upon `the free motions of the plates. It is notnecessary to use cement,y as the spaces between the plates may beca1ked,

but, in rgeneral, the use of cement results in a better assemblymechanically.

As noted above, the gasket 5 is compressible to permit adjustment of thescrews 3 to obtain that clamping pressure which affords the 'maximum andYsmoothest frequency response.

The four plates or double plates of the diaphragin are u,connected inparallel, as shown in Fig. 4, care being taken that the connections areso made/that the motions of the four units are in phase. The clearancebetween the throat and the surfaces of the crystals was of the order of0.010 inch. v

The performance of a loud speaker of this type and designed for thereproduction of the higher audible frequencies is shown by curve E ofFig. 5. The data for this curve was obtained by measuring the pressureinthe sound wave emanating from the horn for applied voltages ofdifferent frequencies. across the crystals for all frequenciesV and thepressuresV were measured on the axis of the horn at a'distance of threefeet fromvthe outer end.

'The dimensions 'of the crystals were so chosen that elastic resonancewas obtained at a frequency approximately equal'to the upper limit ofthe frequency range to be reproduced. As shown by curve E, the designwas such that the resonant frequency was about 8000 cycles.

For many purposes, it is desirable to have a relation between soundpressure and applied voltage which is substantially uniform over an ex-Theslame voltage was maintained- 2,1 oa,ess

tendedl range of frequencies. This characteristic maybe obtained with ahorn having the inherent characteristic oi' Fig. 5 by an appropriatedesign of the circuit which transmits' the voltage to the loud speaker.

vAs shown in Fig. 6a, the generator G or source of power supply isrepresented as having an internal resistance R, andthe circuit completedbetween the terminals of the generator G and the4 loudspeaker C includesthe series inductance L which tunes the circuit-'to a frequency lowerthan the resonant frequency ofthe loud speaker. With this' arrangement,the sound pressure response will beesubstantially uniform for allfrequencies between the resonant frequency ofthe circuit and theresonant frequency of the horn.

' When it is desired to secure an impedance match with a generator ofarbitrary internal resistance, the speaker Cmay be/connectedto thesupply circuit through a transformer T, Fig. 6b, which transformercorresponds to the transformer lof Fig. 1. With this arrangement, thetuning inductance L may be inserted in the primary, as shown.

or in the `secondary circuit in series with the horn. It may` also bebuilt into the transformer, for example. as leakage inductance.

The circuit may also be resonated by means y of a shunt inductance L',or equivalent reactance, as shown in Fig. `6c.

The eiect of the series tuning inductance L upon the sound pressurecharacteristic is illustrated graphically in Fig. 7, the several curvesbeing identified by legends L=0.35 mh, etc., to indicate the value, inmillihenries, of the inductance effectively included in the supplycircuit when the data for a particular curve was obtained.

The electrical supply circuit may also be tuned to a frequency higherthan the resonant frequency of the loud speaker. In this casetheresponse tends to become more uniformabove the diaphragm resonantfrequency rather than below it.

Some control over the sound-pressure frequency characteristic may alsobe obtained without -the tuning inductor L by adjusting the effectivecapacity reactance of the loudspeaker relative to the internalresistance of the supply source. The most convenient way todo this is bymeans of. a transformer inserted between the supply source and loudspeaker. Increasing the secondary-primary turns ratio will in generalcause an increase in the effective crystal capacity and a greaterslumping oi! of voltage across the crystais with frequency.

The above data is given merely as an illustration of the operatingcharacteristic of.one particularembodiment, and it is to be understood'that the'invention is not limited to any particular frequency range ofoperation. By appropriate design of the diaphragm and the associatedelectrical system, the frequency range of reproduction may be locatedatanother section of the audio frequency range, and it may be increased ordecreased in width in accordance with the design requirements for aparticular construction.

I claim;

1. In an electromechanical transducer, ,a flat assembly of fourpiezoelectrically active plateshaped elements arranged in edge alinementand with corners of the said elements substantially meeting at a commoncentral point of said assembly, each of said elements comprising a pairof superposed crystal plates having-an `electrode between their adjacentfaces and electrodes at arcanos' their outer faces, and circuitconnections between the several electrodes of said elements toproducelike deiections oi all elements atsaid common point when voltages areimpressed across said electrodes or, alternatively, to produce likeelectrical responses between connected electrodes of all elements `whenpressure is applied to said common point of the assembly, and meansintroduced between the contiguous edges of said elements to preventleakage of air past said contiguous edgessaid 'means consisting oi aresilient cement joining the adjacent edges of said elements to eachother.

2. The invention 'as claimed ln claim l, in com` bination with meansengaging marginal areas at opposed faces of said at assembly of elementsfor limiting movement oi the said engaged areas. 3. 'A loud. speakercomprising a horn, a diaphrafgm comprisinga plurality ofpiezoelectrilcally active crystal plates secured across the throataofsaid horn, and sealing means between and cooperating withsaid diaphragmand horn to form an air-tight throat chamber.

4. A loud speaker comprising a horn, a dia phragm comprising a pluralityof piezoelectrically active crystal plates at the throat of the horn,

a gasket cooperating with said throatand horn the throat of the horn, apiezoelectrically' active diaphragm positioned adjacent the throat oi'the horn,a gasket between said diaphragm and`horn base, and meansclamping said diaphragm-against said gasket, whereby said gasketcooperates with 11:; said 4diaphragm and horn 'base to'iorm an airtightthroat chamber.

' 7. Aloud speaker as claimed in claimt, where-Y in said diaphragmcomprises a plurality of vplateshaped crystal units' havingM theircontiguous edges cemented to each other; each unit beingpiezoelectrically responsive normally toits planeA trdei'ine "an annularopening into said throat chamber, and means comprising -ribs projectingradially from one of said members for' centering said insert memberwithin said horn member.

9. In a loud speaker, a pizoelectrically active diaphragm, a hornprovided with a throat chamhorn member, a vpiezoelectrically. activediaphragm and means mounting the same atthe base o! the horn to' form anair-tight throat chamber, said diaphragm and horn member having anoven-all resonant sound-pressure' frequency characteristic, saidresonantfrequency being locatedat the Aupper end of the range of irequencies tobe reproduced.

l1. :A piezoelectric loud speaker` comprising a horn member, apiemelectrically active diaphragm and means mounting the same at thebase oi' the horn to 'form an air-tight throatv chamber, said diaphragmand horn member having an over-all resonant sound-pressure'frequency`characteristic, ,said resonant frequency being f located above '1,000 T

